Unusual Flexibility of Microporous Sulfides during Ion Exchange

Open-framework chalcogenides with ion-exchange capacity are promising materials for removing hazardous heavy-metal ions and for capturing radioactive Cs⁺. However, research on the exchange mechanism is limited, especially for the framework chalcogenides that have multiple bridging anions. Generally, open-framework chalcogenides that have multiple bridging anions at the window or wall of the channels are rigid during the ion-exchange process. We show here that microporous sulfides with μ₃-S^2- (where μ₃ = triple bridging mode) at the windows exhibit framework flexibility upon ion exchange. Three new microporous sulfides Na₄Cu₈Ge₃S₁₂·2H₂O (1), Na₃(Hen)Cu₈Sn₃S₁₂ (where en = ethylenediamine) (2) and (dap)₂(Hdap)₄Cu₈Ge₃S₁₈ (where dap = 1,2-diaminopropane) (3) were synthesized under solvothermal conditions. Compounds 1 and 2 contain a copper-rich framework composed of icosahedral [Cu₈S₁₂]^16- units linked via monomeric GeS₄^4- or SnS₄^4- tetrahedral units, whereas compound 3 features an expanded framework composed of icosahedral [Cu₈S₁₂]^16- units interconnected with dimeric Ge₂S₆^4- units. These compounds exhibit unusual ion-exchange properties. Specifically, the frameworks of 1 and 2 (with μ₃-S at the small windows) show "breathing action" upon ion exchange of K⁺ or Rb⁺, which have relative large sizes, and compound 3 exhibits framework flexibility upon Cs⁺ ion exchange with both space group and channels changed.